This invention relates generally to a machining system and method and more particularly to a new and improved real-time machining and on-machine inspection system incorporating in-process measurement of selected workpiece dimensions, such as the diameter, and directly controlling the path of the cutting tool to provide for the actual final machining of the workpiece or part to the desired dimensions prescribed by a part program.
Computer controlled machine tools which employ a computer numerical control (CNC) wherein the path of the cutting tool is directed in accordance with signals from the computer as the part program is executed are now well known and widely used. However, such known computer controlled machine tools are not always entirely satisfactory especially for automatically machining workpieces which have a narrow tolerance range. That is, such known systems are not capable of automatically machining a workpiece to assure that the final dimensions of the actually machined workpiece conform to the part program. This may result from tool wear, imperfect alignment of the workpiece in the machine tool, an offset of the cutting tool bit, thermal growth, differences in cutting tools, or the like. For example, if the workpiece is mounted in the machine tool so that its center line and the center line of the headstock and the center line of the tailstock are not perfectly in line, then with the conventional computer controlled machine tool a cylindrical workpiece will be machined with a taper. This defect can only be determined after machining by inspection, such as by hand gaging followed by remachining of the workpiece.
Methods are known in the prior art to reduce the adverse effects of tool wear and thereby improve the operation of the machining process. One such method, for example, is described in U.S. Pat. No. 4,620,281 wherein the condition of the cutting tool is monitored during a turning operation and the machine is adjusted to compensate for tool wear. The device can also be used as an in-process part inspection system. In that patent a probe on the cutting tool-tool holder assembly measures the distance to the freshly cut surface and the probe signal is monitored; when this distance changes indicating tool nose wear, the numerical control is alerted to adjust the tool during the chip-forming operation to compensate for wear. A broken or worn out tool signal shuts down the machine. After a cutting pass, the part is inspected by retracting the cutting tool and running the part program. When the measured distance is not constant, there is error in the part.
Such prior art methods can still not provide a machining system which can assure that the final profile and dimensions of the actually machined part conform to the part program within the required tolerance, which in turn conform to the intended part design. For example, the part may not be centered in the machine and since the prior art method measures only the gap between the gauge and the part these errors are indistinguishable by such methods.
The present invention for the first time provides a system and method to reliably provide for real-time, in-process measurement of actual dimensions of the workpiece during the machining operation, as well as to provide for in-process inspection of the workpiece without requiring the workpiece to be removed from the machine.